When women live together, do their menstrual cycles tend to synchronize? It’s been a long time since I first heard that claim. I didn’t believe it, for a number of reasons. I had never observed it myself, I saw no plausible mechanism to explain how it could happen, I thought the statistics to prove it would be problematic and complicated, and I suspected that confirmation bias and selective memory might have persuaded people that a spurious correlation existed. How often do women say “Oh, look! We’re having our periods at the same time”? How often do they say “Oh, look! We’re having our periods at different times”? Now that many years have passed since my first encounter, I thought it would be fun to revisit the claim and see whether science has supported it or rejected it.

A perusal of PubMed and other Internet sources left me confused and amused. (more…)